Epitaxially Grown PtxIr100-X Alloys with Enhanced Properties for Ammonia Electro-Oxidation

Ammonia is a highly toxic gas, a major environmental pollutant, and a potential fuel for fuel cells. Thus the electro-oxidation of NH₃ has important applications in NH₃ safety sensors, NH₃ decontamination in wastewater, and power production. However, on most electrode surfaces, the electro-oxidation of NH₃ is plagued with low surface reactivity and surface contamination by reaction intermediates.

The vast majority of studies on the NH₃ oxidation reaction have been performed on Pt electrocatalysts, and Pt remains the electrocatalyst which shows the highest current density at the lowest overpotential for catalyzing the oxidation of NH₃ to N₂. On Pt, the oxidation of ammonia occurs predominantly on (100) terraces. However, to the best of our knowledge, the effect of strain in the outmost metal layers and hybridization of the Pt d-states with Ir atoms (ligand effect) on the electro-oxidation of NH₃ have never been studied. Accordingly, we have investigated the epitaxial growth Pt_xIr_100-x alloys model electrocatalysts that mimic single crystalline systems. This was achieved by deposition on MgO(100) via pulsed laser deposition.

It will be shown that Pt and Ir form a kinetically stable alloy and that the Pt_xIr_100-x alloys grew with the desired PtIr(100)[010]//[010](100)MgO configuration over the whole composition range. Also, the XPS data show that a surface alloy is formed over the whole composition range.

Electrochemical measurements were performed in 0.5 M H₂SO₄ to highlight the preferential (100) surface orientation. Cyclic-voltametry and chrono-amperometry experiments in alkaline media were then carried out in presence of ammonia. They demonstrate the importance of the (100) surface structure for the electro-oxidation of ammonia compared to randomly oriented surface (polycrystalline sample). The presence of iridium decreases the surface poisoning at PtIr (100) alloys. Finally, it will be shown that surface poisoning can be further reduced using a simple cyclic-voltammetry procedure.